Ultrapure polysilicon used in the electronic and solar industry is often produced through deposition from gaseous reactants via a chemical vapor deposition (CVD) process conducted within a reactor.
One process used to produce ultrapure polycrystalline silicon in a CVD reactor is referred to as a Siemens process. Silicon rods disposed within the reactor are used as seeds to start the process. Gaseous silicon-containing reactants flow through the reactor and deposit silicon onto the surface of the rods. The gaseous reactants (i.e., gaseous precursors) include silicon halides such as trichlorosilane mixed with a suitable carrier gas, generally hydrogen. Because trichlorosilane is kinetically stable, CVD processes are rather slow and commonly utilize relatively high temperatures to permit the deposition to occur. It is not uncommon to utilize rod surface temperature greater than 1000° C. Under such conditions the gaseous reactants decompose on the surface of the rods. Silicon is thus deposited on the rods according to the following global reaction:SiHCl3+H2→Si+3HCl.
The process is stopped after the rods grow to the desired diameter. The rods are then extracted from the CVD reactor and the silicon is harvested from the rods for further processing.
During the CVD process, the surface temperature of the silicon rods typically needs to be controlled. If the surface temperature is too high, excessive silicon dust may be produced. If the surface temperature is too low, the deposition may be slow or may not even occur.
The Siemens process employs Joule heating to achieve desired surface temperatures. Electrical energy is converted into thermal energy to heat up the silicon rods. Electrical current is provided to the reactor by a power supply that adjusts the voltage supplied across each rod in order to control the current intensity, and thus the temperature of the rod.
During the deposition process, however, the power demand of the reactor is not constant. The heat flux leaving the silicon rods increases with the deposition time as the surface area of the rod increases. Accordingly, the current through the rods is constantly adjusted in order to maintain the desired rod surface temperature.
At least one known method of controlling rod temperature utilizes a pyrometer to monitor the rod surface temperature. When the monitored temperature deviates from a desired set point, electrical intensity is adjusted to attempt to return the rod surface temperature to the desired set point. Pyrometers generally determine the temperature of a target spot on a silicon rod from the intensity of the radiation emitted at a certain wavelength or range of wavelengths according to Planck's radiation law.
However, pyrometers must be properly calibrated and aimed to a suitable target which preferably is a smooth homogeneous spot of the silicon rod of interest. Furthermore, monitoring rod surface temperature using a pyrometer imposes other difficulties. These difficulties include the presence of local hot or cold spots, pyrometers aimed out of target, and movement of a rod out of target, among other possible difficulties.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.